In general, rotary power tools are light-weight, handheld power tools capable of being equipped with a variety of accessory tools and attachments, such as cutting blades, sanding discs, grinding tools, and many others. These types of tools typically include a generally cylindrically-shaped main body that supports a drive mechanism and often serves as a hand grip for the tool as well. The drive mechanism includes an output shaft that is equipped with an accessory attachment mechanism, such as a collet, that enables various accessory tools to be releasably secured to the power tool.
Accessory tools for rotary power tools typically have a work portion and a shank. The work portion is configured to perform a certain kind of job, such as cutting, grinding, sanding, polishing, and the like. The shank extends from the work portion and is received by an accessory attachment system on the power tool. The accessory attachment mechanism holds the shank in line with the axis of the output shaft so that, when the output shaft is rotated by the motor, the accessory tool is driven to rotate about the axis along with the output shaft.
The output shaft is rotated at very high speeds when driving an accessory tool to perform work. The accessory tools and accessory attachment mechanisms for rotary tools are designed for operation up to a pre-specified maximum limit without losing integrity or falling apart. As an example, accessory tools for a rotary tool may have a rated limit of 35,000 rpm above which the accessory tools should not be operated.
Some rotary tools, however, are capable of rotational speeds that exceed the rated limit of the accessory tools with which they are configured to operate. In these cases, speed limiting mechanisms and systems are incorporated into the drive system of the tool to ensure that the rated limit of the accessory tools is not exceeded. For electrically powered tools, the rotational speed of the electric motor may be easily controlled and limited in a variety of ways through the design of the circuitry. For pneumatically powered tools, however, speed limit control must be implemented in other ways. Therefore, one issue faced in the design of pneumatically-powered rotary tools is coming up with effective and efficient means for limiting the speed of the rotary tool without hampering or adversely impacting performance.